This invention relates to apparatus and methods for chemically etching a workpiece using a process gas.
It has been known for many decades that when plasma etching a workpiece, it is possible to detect the point at which an etch has cut through a first layer and reached the underlying layer by the change in the optical emission resulting from the change in the exhaust or reaction products being generated and then being ionized by the plasma. Endpoints can also be detected by changes in consumption of etch gas, which can also be detected optically. Initially these emissions were monitored by skilled technicians. Subsequently the process was automated using optical end point detectors.
Endpoint processes based on the first part approach can only work where there is a plasma being generated and the exhaust and reaction products enter the plasma. There have been, accordingly, a number of proposals, such as that described in U.S. Pat. No. 4,857,136, in which the exhaust products from the turbo pump are ionized and the resultant optical emissions monitored. Experience has shown that arrangements of this sort can have a sluggish response and repeatedly is difficult to achieve due to limitations in local conditions.